1. Field of the Invention
The present invention relates to transfer paper for electrophotography, which is used in indirect dry-type electrophotographic full-color duplicators and printers. Precisely, it relates to coated transfer paper for electrophotography having the advantages of high glossiness, no blister in image fixation through simplex and duplex printing, and high-quality image formation.
2. Description of the Related Art
The recent tendency in the art is toward high-speed duplication in duplicators and printers to give high-quality color images. In particular, in the field of on-demand publications, color duplicators and color printers are being much used for producing publications which have heretofore been produced in an ordinary printing method, as being relatively easy to use for producing a small number of publications.
In general, conventional coated paper with high glossiness is produced by applying a pigment having a mean particle size of at most 2 microns onto base paper in an amount of at lest 10 g/M.sup.2 for each side, through coating with various coaters, followed by smoothening the coated surface through calendering. The coated paper with high glossiness is generally used in the field of commercial printing, but is being much used in electrophotographic duplicators and printers, in place of ordinary PPC paper or copying paper, to increase the sharpness of images to be printed thereon.
However, using the coated paper with high glossiness in electrophotographic duplicators and printers is problematic in that it is often blistered while toner images are fixed thereon under heat. Blistering as referred to herein includes the following: While toner images are fixed on paper under heat, water existing in the paper is heated to vaporize, whereby the water vapor pressure in the paper increases. The water vapor is discharged outside the paper, but if not smoothly discharged for some reasons, it will rapidly expand inside the base paper to give local blisters. This is hereinafter referred to as paper blistering.
Another is as follows: The high-pressure water vapor having been formed inside the paper shall be discharged out through the void in the coating layer. However, if toner images are on the paths through which the water vapor is being discharged out, the water vapor will break the toner images to flow outside. In that condition, the coated paper shall have fine blisters or through-holes, is thereby degraded. This is hereinafter referred to as toner blistering. The toner blistering produces serious image defects.
Owing to its coating layer, the coated paper could not ensure sufficient water vapor discharging paths. When having toner images thereon, it is often troubled by paper blistering and toner blistering. In particular, the coated paper with high glossiness requires calendering for increasing its glossiness. However, calendering increases the density of the coating layer, and the calendered coated paper will be much more troubled by paper blistering and toner blistering.
The blistering occurs more frequently in duplex printing than in simplex printing. This is because, in duplex printing, toner adheres first on one surface of the coating layer to form toner images thereon, and thereafter on the other surface thereof also to form toner images thereon. In the latter step of such duplex printing, the water vapor having been formed in the paper is more difficult to go out than that to go out in simplex printing. Conventional coated paper with high glossiness for ordinary printing and that for electrophotographic printing are blistered only a little or are not blistered in simplex printing, but are significantly blistered in duplex printing.
For overcoming blistering of coated paper for electrophotography, some methods have heretofore been proposed. One comprises controlling the center line average height of the surface of the coating layer to be at most 2.0 .mu.m and controlling the air permeability of the coating layer to be at most 4000 seconds (see JP-A 62-198876); and another comprises controlling the smoothness and the air permeability of the base paper to be coated and specifically defining the amount of the coating layer that contains an organic pigment to thereby control the air permeability of the coating layer to be at most 4000 seconds (see JP-A 5-241366). The air permeability is measured according to an Ohken's air permeability test method stipulated in Japan Tappi No. 5 (this accords with JIS P8117), in which is measured the time (in terms of seconds) as taken by 10 ml of air passing through paper under a pressure of 0.1 kg/cm.sup.2, and the time thus measured indicates the vapor permeability of the paper tested.
However, those types of coated paper for electrophotography proposed as above are still problematic in that they are often troubled by paper blistering and toner blistering in duplex printing though little in simplex printing. The reason will be because, in simplex printing, water vapor discharge from the toner image-formed surface of the paper is difficult but is easy from the opposite surface thereof, and therefore the printed paper is blistered little. However, in duplex printing, the toner images formed shall cover the both surfaces of the paper, and the vapor permeability of the paper will be not enough for smooth water vapor discharge in that condition.